1. Field of the Invention
The present invention relates to high speed printers capable of printing up to at least 350 pages per minute.
2. Description of the Prior Art
Prior art printers which store a digital image of information to be printed are generally of two different types. The first type is based in hardware which processes a page very quickly to implement standard printing functions but is not capable of a wide diversity of printing functions. The assignee's Mercurion printer is a high speed printer utilizing a hardware base which permits printing up to 80 pages per minute. The second type of printer is based upon a microprocessor control which is capable of a high degree of programmability to implement different printing functions but does not operate at high speeds. Systems of this type typically have a throughput of between 10 and 20 pages per minute.
Neither of the foregoing types of prior art systems utilize a processing unit which issues commands to a character generator to retrieve characters stored as a pixel matrix of words of pixels in a font storage memory with all addresses for fetching the words of the characters from the font storage memory being generated independently of the operation of the processing unit, which processes words of each retrieved character word to position the words at particular pixel locations with respect to word boundaries in a page memory where the words are to be stored, and which specifies only the absolute address of the page memory where the first word of each character is to be stored with all of the absolute addresses in the page memory where the remaining words of each character are to be stored being generated independently of the processing unit. The prior art microprocessor based systems have the control of all operations for supporting the system assigned to the microprocessor including the generation of addresses which produces a slow throughput when compared with the present invention which utilizes the processing unit to perform functions in supporting the operation of the system without bearing the substantial overhead of generating addresses to operate the system memories.
The prior art systems for duplex printing (printing on both sides of a page) have a font storage memory storing four separate character orientations for each of the characters to be printed. The first orientation is for characters used in the portrait mode (lines running across the width of the paper) which is represented by the letter "A" of FIG. 1A and the second orientation is for characters used in the landscape mode (characters running across the length of the paper) which is represented by the letter "A" of FIG. 1B. The portrait mode is used for printing characters on one side of a page in books and similar types of printing applications. The landscape mode is used for printing characters in presenting graphical data and other types of information on one side of a page where it is desirable to have the textual material running in lines across the length of the paper. The third and fourth orientations are respectively 180.degree. rotations of the aforementioned first and second orientations which are useful for printing the backside of pages in the "duplex" mode of printing for the aforementioned portrait and landscape orientations and are respectively represented by the letter "A" in FIGS. 1C and 1D.
While not illustrated in FIGS. 1A-1D, it should be understood that each character of each font is stored as a matrix of pixels which are readout in a manner similar to a raster of a television scanning pattern with the individual pixels representing points within the raster which are stored as binary values with one binary value indicating a light area and the other binary value indicating a dark area with the net effect of all of the binary values creating the visual appearance of the character. The disadvantage of the prior art method of storage of characters, as illustrated in FIGS. 1A-1D to implement both "simplex" (printing on only one side of a sheet) and duplex printing, is that each particular character must be stored in four separate orientations which requires double the amount of character memory for only simplex printing.
Systems are known for rotating stored images or individual characters in multiples of 90.degree. of rotation in display or printing systems. Systems of the aforementioned type are disclosed in U.S. Pat. Nos. 4,271,476, 4,312,045, 4,542,377, 4,545,069, 4,554,638, 4,560,980, 4,570,158 and 4,636,783. U.S. Pat. 4,271,476 discloses a system for rotating a stored digital image 90.degree. from a horizontal to a vertical format. U.S. Pat. No. 4,312,045 discloses a printing system which selectively rotates images 90.degree. . U.S. Pat. No. 4,545,069 discloses a system for selectively rotating digital images by 90.degree. . U.S. Pat. No. 4,544,638 discloses a display system which permits an image to be rotated in multiples of 90.degree. . U.S. Pat. No. 4,560,980 discloses a printer which permits selective rotation of characters by 90.degree. . U.S. Pat. No. 4,570,158 discloses a system which permits an image to be written into memory in either a left to right or a right to left direction and then read out in either a top to bottom direction or a bottom to top direction. U.S. Pat. No. 4,636,783 discloses a system for rotating a stored digital image by multiples of 90.degree. . None of the aforementioned patents discloses the selective readout of stored words in a first direction in which the individual pixels of each word are transmitted in parallel format to a display or printing device in a first order and in a second direction, opposite to the first direction, in which the individual pixels of each word are transmitted in parallel format to a display or printing device with the order of the pixels of each word reversed with respect to the significance of the pixel positions (i.e., the most significant pixel is exchanged for the least significant pixel, the next most significant pixel is exchanged for the next least significant pixel etc.) of the first direction.
U.S. Pat. No. 4,225,929 discloses a system for rotating a television frame 180.degree. about its central point by reversing the order of addressing a stored frame in memory and the order of the readout of the individual pixels. The address generation of U.S. Pat. No. 4,225,929 permits only a full frame to be read out. No provision is made for areas within the frame to be read out which is of extreme importance in a printing system of the present invention where either individual characters are to be read out from a font storage unit in either a first orientation or an inverted orientation or where a part of a page of information or a full page of information stored in a page memory are to be read out in either the first orientation or the inverted orientation for duplex printing of individual sheets or printing of sheets having holes printed on one side for containment in a ring binder.
U.S. Pat. No. 4,555,763 discloses a printer in which a first microprocessor receives, controls and formats character representing digital data and associated control data received from an interface device and a second microprocessor controls selection of characters from stored fonts and dual line storages of data to be displayed. The individual character cells are read in 6 or 8 bit words.
U.S. Pat. No. 4,452,136 discloses a printing system which utilizes two microprocessors. The first microprocessor controls communications with a host system and the second microprocessor controls the operation of a motor driven head which prints a dot matrix.